The present invention relates generally to extrusion devices for plasticating synthetic resinous material. More particularly, the present invention concerns an improved screw for use in connection with such extrusion devices.
With extrusion devices, there has been much interest in obtaining improved mixing of the plasticated material which is extruded. Poor mixing can be the cause of visual imperfections, fluctuating pressure in the material upstream of an extrusion die, temperature excursions in the material and similar phenomena. Moreover, good mixing reduces the amount of unmelted particles which may ultimately reach and partially block a screen frequently used in extruders upstream of the die.
While increasing the rate of energy input to the material can be effected by increasing the rotational rate of the extruder screw, there is an associated increase in temperature of material and localized variations in temperature. Since typical synthetic resinous materials are susceptible to degradation at high temperatures, merely increasing the rotational rate is not a satisfactory solution. Furthermore, the output is almost proportional to the rotational rate, so the energy input per pound of material processed only changes to a minor degree.
Satisfactory mixing of the processed material is of vital importance in most extruding operations, but the quality of mixing is not directly improved by increasing the energy input. What are important to achieve proper mixing are motions which enhance dispersion and distribution of one or more additive ingredients or which homogenize the melt and make the temperature of the melt uniform.
In one effort to reduce the number of unplasticated particles in an extrusion device, a wide intermediate screw flight has been used in conjunction with a helical channel, as disclosed in U.S. Pat. No. 3,701,512. In that device, however, the wide intermediate flight has a substantial width in order to apply a high level of shearing forces to all the material traversing the intermediate flight. Thus, all material, plasticated as well as unplasticated, is subjected to intensive shearing forces. This approach is unsatisfactory in that it unduly increases the temperature of the plasticated material because of the increased mechanical working thereof, and does not promote temperature uniformity in a satisfactory manner.
From an economic standpoint, it is normally desirable to maintain the extruder length as short as possible but yet achieve proper melting and mixing of the material. Extruder length is especially important in multistage extruders wherein the stages are separated by vents to remove volatile materials.
One effort to avoid some of the problems discussed above is disclosed in U.S. Pat. Nos. 3,870,284 and 4,015,832 issued to the assignee of the present invention. In the apparatus disclosed in these patents, an extruder screw is provided with a helical channel having an undulating depth which enhances the mixing characteristics of the extruder screw while providing highly satisfactory pumping or metering characteristics.